Elevated serum cholesterol, particularly in the form of low density lipoprotein (LDL)-cholesterol, is a principal risk factor for cardiovascular disease. The protein component of LDL, apolipoprotein B (ApoB), is secreted from the liver, and the relative efficiency of apoB secretion is an important determinant of the plasma level of LDL. Microsomal triglyceride transfer protein (MTP) plays an important role in apoB secretion. Accordingly, any phenomenon that alters MTP expression or activity may influence apoB secretion and thereby affect serum LDL-cholesterol levels.
MTP is a heterodimer comprising two subunits: (i) an MTP-specific 97 kDa polypeptide and (ii) the multifunctional 55 kDa protein disulfide isomerase (PDI) (Gordon et al., Trends Cell Biol., 5:317-321, 1995). MTP function is absolutely required for assembly and secretion of apoB-containing lipoproteins. Non-apoB-secreting cells can only be converted to apoB secretors if the MTP gene is provided together with the apoB gene (Gordon et al., Proc. Natl. Acad. Sci. (USA), 9 1:7628-7632, 1994; Leiper et al., J. Biol. Chem., 269:21951-21954, 1994). Conversely, inhibition of MTP activity in cells that normally secrete apoB results in a drastic reduction in apoB secretion (Jamil et al., Proc. Natl. Acad. Sci. (USA), 9 3:11991-11995, 1996; Haghpassand et al., J. Lipid. Res., 3 7:1468-1480, 1996). A complete lack of MTP activity, such as, e.g., in cells containing mutations in the MTP coding region, leads to abetalipoproteinemia (Sharp et al., Nature, 365:65-69, 1993; Shoulders et al., Hum. Mol. Gen., 2:2109-2116, 1993; Narcisi et al., Am. J. Hum. Gen., 5 7:1298-13, 1995.
The promoter region of the MTP gene is highly conserved across mammalian species and contains potential control sequences for regulating MTP expression in different cell types and in response to metabolic regulators. Transcriptional activation of the human MTP promoter is suppressed by insulin and enhanced by cholesterol (Haoan et al., J. Biol. Chem., 269:28737-28744, 1994). The insulin response has also been demonstrated in HepG2 human liver carcinoma cells (Lin et al., J. Lipid Res., 3 6:1073-1081, 1995). It has also been shown that liver cells in hamsters fed either a high-fat or a cholesterol-enriched diet contain higher concentrations of MTP mRNA.
The high morbidity and mortality associated with cardiovascular disease means that there is a need for methods and compositions that allow the identification of individuals who are at higher or lower risk for developing cardiovascular disease. There is also a need for methods and compositions that allow the identification of treatment regimens that result in the most positive therapeutic outcome.